Magnetic filtration method for minimizing radioactive waste during abrasive waterjet cutting

ABSTRACT

A method of separating radioactive cut material from non-radioactive abrasive cutting material is disclosed herein. The method includes the steps: determining a magnetic susceptibility of a radioactive material to be cut; selecting a magnetic susceptibility of a radioactive material to be used with a water jet cutting tool such that the abrasive cutting material has a different magnetic susceptibility than the radioactive material to be cut; capturing a mixture of radioactive cut material and abrasive material generated during operation of the water jet cutting tool; and separating the radioactive cut material from the abrasive cutting material using a magnetic filter.

RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 09/265,824 filed on Mar. 10, 1999, now U.S. Pat. No. 6,156,194,and claims the benefit of U.S. Provisional Application Ser. No.60/093,796 filed on Jul. 23, 1998. The subject matter of thisapplication is related to the Applicant's copending applications titled“Filtration System for Concentrating Radioactive Debris,” “Container forHandling and Storing Radioactive Debris,” and “System and Method ofSealing Container for Handling Radioactive Debris” (Application Ser.Nos. 09/265,823; 09/265,826 and 09/265,825, respectively), all of whichare being filed concurrently with the present application and areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to abrasive waterjet cutting anddisposal of radioactive material. In particular, the present inventionrelates to systems and processes that separate waterjet abrasivematerial from high level radioactive material cut by an abrasivewaterjet process to minimize the amount of high level radioactivematerial for handling and disposal.

2. Description of the Related Art

Abrasive waterjet cutting is a process where a high pressure watersource is forced through a nozzle and mixed with an abrasive medium. Thehigh speed jet of water is dynamically unstable and breaks into smalldroplets that accelerate the solid abrasive particles. This highvelocity abrasive slurry micro-machines a workpiece by causing erosion,shearing and failure due to rapidly changing localized stresses. Theworkpiece particles and abrasive particles are mixed as a result of thecutting process.

The abrasive waterjet cutting process requires several times the massand volume of abrasive particles to perform the cut than are removedfrom the workpiece in the kerf of the cut. Therefore, when cuttingradioactive materials, the mixture of the abrasive particles andworkpiece particles represents an increased volume of high levelradioactive material that must be handled and stored.

Waterjet cutting processes are commonly used during the dismantling ofnuclear reactors. For example, waterjet cutting processes are often usedon radioactive components, such as nuclear reactor internals, vessels,and other activated or contaminated materials and structures. Theseradioactive components are normally cut using a waterjet cutting processin a reactor vessel cavity, spent fuel pool or other underwater pool orcell.

The mixture of waterjet abrasive material and high level radioactivematerial removed during watejet cutting processes is considered unstableunless it is captured and stored in high integrity containers. Thehandling and storage of large volumes of such radioactive material isexpensive and takes up a large amount of the limited space available forstoring the material. Thus, there is a need for a system that separatesnon-radioactive abrasive material from the high level radioactivematerial so that the materials can be disposed of separately, therebyreducing handling concerns and disposal costs.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a system forseparating a sufficient amount of radioactive material fromnon-radioactive material in a fluid stream to reduce the volume and costof disposal of radioactive waste.

It is a further object of the present invention to provide a magneticfiltration system that removes a magnetic material from a fluid streamcontaining both magnetic and nonmagnetic waste materials to reduce thevolume and cost of disposal of radioactive waste.

It is yet a further object of the present invention to provide a systemthat separates waterjet abrasive material from radioactive material cutby an abrasive waterjet process, whereby the radioactive material can behandled and stored separately from the abrasive material to minimizehandling and disposal costs.

It is a still further object of the present invention to provide amagnetic filtration system that removes both abrasive material andradioactive cut material from a fluid stream using a magnetic filterwhere differentiation between the materials based on magneticsusceptibilities is not possible or practical, thereby minimizingsecondary waste caused by the use of consumable cartridge filters.

According to a broad aspect of the present invention, a magneticfiltration system is provided for separating radioactive cut materialfrom a fluid stream containing a mixture of the radioactive cut materialand a nonradioactive abrasive cutting material. The system comprises amagnetic filter in fluid communication with the fluid stream. Themagnetic filter has a first outlet for discharging material having ahigh magnetic susceptibility and a second outlet for dischargingmaterial having a low magnetic susceptibility. By selecting an abrasivecutting material having a different magnetic susceptibility than theradioactive material being cut, the magnetic filter can separate theabrasive cutting material from the radioactive cut material.

The abrasive cutting material can be a cast steel, stainless steel, shotor other material having a high magnetic susceptibility relative to theradioactive material being cut. Alternatively, the abrasive cuttingmaterial can be a garnet, alluvial or other material having a lowmagnetic susceptibility if the radioactive material being cut has a highmagnetic susceptibility. In cases where the abrasive and the radioactivematerial being cut both have high magnetic susceptibility, the magneticfilter can be used to capture both the abrasive and radioactivematerial, thereby minimizing or eliminating secondary waste caused bythe use of consumable cartridge filters.

According to another broad aspect of the present invention, aclosed-loop filtration system is provided for removing radioactiveparticulate from a volume of fluid. The filtration system comprises: anenclosure submerged in a fluid volume for isolating a portion of thefluid volume from a remainder of the fluid volume; a capture structurefor capturing abrasive cutting material and radioactive cut particulategenerated within the enclosure; and a separating arrangement in fluidcommunication with the capture structure for receiving a fluid streamcontaining the abrasive cutting material and the radioactive cutparticulate, the separating arrangement comprising a filter forseparating the abrasive cutting material from the radioactive cutparticulate, and an outlet that returns the fluid back into theenclosure.

The closed-loop filtration system according to this aspect of theinvention has a magnetic filter for separating the abrasive cuttingmaterial from the radioactive cut material. The abrasive cuttingmaterial is selected so that it has a different magnetic susceptibilitythan the radioactive material to be cut, thereby allowing the magneticfilter to separate the high level radioactive materials from the lowlevel abrasive materials for separate handling and disposal.

A filtering arrangement is placed in fluid communication with themagnetic separating arrangement. The filtering arrangement comprises aplurality of filters that remove the radioactive cut particulate fromthe fluid stream in multiple stages according to a size of theparticulate. After filtering, the fluid from the fluid stream isreturned to the isolated portion of the fluid volume in a closed-loopfashion.

According to another broad aspect of the present invention, a method ofseparating radioactive cut material from nonradioactive abrasivematerial is provided, comprising the steps of: determining a magneticsusceptibility of a radioactive material to be cut; selecting anabrasive material to be used with a waterjet cutting tool such that theabrasive material has a different magnetic susceptibility than theradioactive material to be cut; capturing a mixture of radioactive cutmaterial and nonradioactive abrasive material generated during operationof the waterjet cutting tool; and separating the radioactive cutmaterial from the nonradioactive abrasive material using a magneticfilter. The step of selecting an abrasive cutting material comprisesselecting an abrasive cutting material with a high magneticsusceptibility if the radioactive cut material has a low magneticsusceptibility, and selecting an abrasive cutting material with a lowmagnetic susceptibility if the radioactive cut material has a highmagnetic susceptibility.

Numerous other objects of the present invention will be apparent tothose skilled in this art from the following description wherein thereis shown and described a preferred embodiment of the present invention,simply by way of illustration of one of the modes best suited to carryout the invention. As will be realized, the invention is capable ofother different embodiments, and its several details are capable ofmodification in various obvious aspects without departing from theinvention. Accordingly, the drawings and description should be regardedas illustrative in nature and not restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more clearly appreciated as thedisclosure of the invention is made with reference to the accompanyingdrawings. In the drawings:

FIG. 1 is a schematic of a closed-loop filtration system for filtering,concentrating and transporting radioactive debris generated from anabrasive waterjet cutting operation; and

FIG. 2 is a schematic of a closed-loop filtration system equipped with amagnetic filter for separating radioactive material from nonradioactivematerial according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A magnetic filtration system 100 for separating, filtering,concentrating, and transporting radioactive debris particulate accordingto a preferred embodiment of the present invention will be describedbelow with reference to FIG. 2. However, first the Applicant's relatedfiltration system 10, which does not include a magnetic filter, will bedescribed with reference to FIG. 1 of the drawings.

The filtration system 10 uses a cut zone enclosure 11 submerged in awater volume 12 for isolating a portion of the water volume from aremainder of the water volume. The cut zone enclosure 11 provides anarea in which an abrasive waterjet cutting tool 13 can be operated, forexample, during the dismantling of nuclear reactor internals. The cutzone enclosure 11 keeps the radioactive particulate generated during thecutting process separate from the remainder of the water volume 12 toprevent or minimize contamination of the general pool area water. Afloating hood 14 is placed over the cut zone enclosure 11 and has anoutlet 15 in communication with a HEPA gas filtration system 16 forpurging gas from an area above the cut zone enclosure 11.

A header 17 having a suction opening 18 is provided in the cut zoneenclosure 11 to draw contaminated water from the cut zone enclosure 11.A capture shroud 19 surrounds the suction opening 18 to facilitate thecapture of radioactive debris particulate from a desired area within thecut zone enclosure 11. The radioactive debris particulate contains amixture of waterjet abrasive material and high level radioactivematerial cut by the abrasive waterjet process.

A flushing pump module 20 has a flushing pump 21 that creates a suctionfor drawing contaminated water from the suction opening 18 within thecut zone enclosure 11. The flushing pump module 20 also contains aseparator 22 that removes large size particulate, such as chips or slagfrom thermal processes, from the contaminated water. The separator 22includes a settling tank, a cyclone and/or a screen on the inlet toseparate out the large size particulate. The separated large sizeparticulate is moved from the separator 22 into a transfer canister 23.The transfer canister 23 provides a means to remotely move the largesize particles into disposal containers/casks used to remove the wastefor burial or storage elsewhere. The settling tank, cyclone, and/orscreen of the separator 22 are sized such that the large size particlesremoved by the separator 22 are of a size that can be mixed and disposedwith solid waste, such as larger segmented parts of the reactor vesselor internals or other components, without the need for addedstabilization. Thus, the large size particles removed by the separator22 of the flushing pump module 20 need not be encapsulated by binding ina solid matrix, or sealing in separate high integrity containers, asrequired by either the Code of Federal Regulations, local or stateregulatory requirements for certain smaller size radioactive particles.

A water flow out of the flushing pump 21 of the flushing pump module 20(now absent the larger size particles) is directed into a solid mediafilter module 30. The solid media filter module 30 contains a pluralityof solid media filters 31 (a commercially available product) thatconcentrate medium size particles, i.e., typically in the size range of15 to 50 microns. Within the solid media filters 31, the water flowtravels from the inside to the outside of a fine metal mesh matrix(i.e., strainer). Particles that will not pass through the mesh areconcentrated in the bottom of a vessel within the solid media filters 31by a slowly reciprocating wiper piston 32. On either a preset pressuredrop or timing cycle, a controller 33 momentarily opens a dump valve andejects the concentrated particles from the solid media filters 31.

A concentrate handling module 40 receives the concentrated particulateejected from the solid media filters 31. The concentrate handling module40 has a purge tank 41 maintained at a pressure sufficiently below aninternal pressure of the solid media filters 31 to provide the pressuredifferential necessary for ejecting the concentrate from the solid mediafilters 31. A transfer pump 42 intermittently moves the concentratedslurry from the purge tank 41 to a disposal canister fill and dewateringstation 50 (“disposal canister station”).

The disposal canister station 50 is located beneath the surface of thewater volume 12 to provide radiological shielding to personnel.Canisters 51 suitable for confining the concentrated particulate areremotely filled through a fill line 52 and dewatered through adewatering line 53 by means of a filter 54 inside the canister 51. Avacuum pump 55 and/or pressure source (e.g., air, nitrogen, or inertgas) assist the dewatering process with the removed water being returnedthrough the dewatering line 53 to the cut zone enclosure 11 afterpassing through a polishing filter 56 (typically 0.3 to 0.5 micron),thereby maintaining a closed-loop system. A hood 57 is placed over thedisposal canister station 50 and has an outlet 58 in communication witha HEPA gas filtration system 59 for purging gas from an area above thedisposal canister station 50.

Small size particles (typically less than 15 to 50 microns), which arenot removed from the water as concentrate in the solid media filtermodule 30, are directed to a cartridge filter module 60. The cartridgefilter module 60 includes a plurality of cartridge filters 61, 62 sizedto handle the full flushing flow rate from the solid media filter module30. The cartridge filters 61, 62 preferably include at least one coarsefilter 61 and at least one find filter 62 for removing particles in atleast two stages based on a size of the particles. The water flowleaving the cartridge filter module 60 is redirected to the cut zoneenclosure 11 to maintain a closed-loop system.

The filtration system 10 includes several design features to facilitateoperation and maintenance, and to reduce the radiation dose topersonnel. For example, the equipment of the filtration system 10 ispackaged in modules as described above to allow easy removal andreplacement of each module or station separate from the other componentsof the filtration system. The equipment also includes remotely operabledisconnects and isolation valves on each module and between majorcomponents within the modules to facilitate removal and replacement.

Instrumentation including a flow meter 70 and pressure transducers 71with remote indicators 72, 73 are included to monitor the performance ofthe filtration system 10. The system contains sufficient redundancy toeliminate or minimize the need to stop cutting operations for filtrationsystem servicing, including the filling and handling of the wastecanisters 23, 51. The system can be operated with a minimum ofattendance, thereby reducing the radiation dose to operating personnelin the pool area 12.

The filtration system according to the present invention has thefollowing advantages over systems that are commonly used during thesegmentation of activated reactor vessel internals and components.

The filtration system 10 is a closed or nearly closed system, whichprevents or minimizes contamination of the general pool area water 12.This system approach: reduces the dose to operating personnel byconfining contamination; reduces cross contamination of other equipment;minimizes the post segmentation water processing complexity and expensesince the volume of highly contaminated water is minimized and can betreated separately and in less time; and reduces the complexity andexpense of decontamination of the general pool area walls and radiationdose to operating personnel once the water is drained.

The volume of secondary radioactive waste is minimized by separation ofparticle sizes. The large size particles, which are large enough to bedisposed of directly with solid waste, are removed first by theseparator 22. The solid media filters 31 remove additional medium sizeparticles through concentrating and purging to a handling system 40, 50that directly fills high integrity disposal canisters 51 that meetstabilization criteria. The low flow rate of the concentrated flow tothe disposal canister station 50 allows using a minimum size dewateringfilter 54 in the disposal canister 51. The cartridge filters 61, 62,which form the major part of secondary waste, are minimized since largesize particles are removed by the flushing pump module 20 and mediumsize particles are removed by the solid media filter module 30, therebyleaving only a small mass of particles that the cartridge filter module60 must remove.

The magnetic filtration system 100 according to the present inventionwill now be explained in detail with reference to FIG. 2. The filtrationsystem 100 has many of the same components/modules as the filtrationsystem 10 shown in FIG. 1 and described above. These samecomponents/modules are identified by the same reference numerals in bothdrawings.

The magnetic filtration system 100 includes a magnetic filter 101 thatseparates the abrasive material used by the waterjet cutting processfrom the radioactive particles of the workpiece material being cut.Radioactive materials cut by the abrasive waterjet process will normallybe stainless steel or high or low alloy carbon steel. These materialspossess different magnetic susceptibilities that can be used todifferentiate between the radioactive workpiece material and theabrasive waterjet material.

According to the present invention, the waterjet abrasive will beselected based on the material being cut. For high or low alloy carbonsteels, which have a high magnetic susceptibility, a typical garnet,alluvial material, or other conventional abrasive material having a lowmagnetic susceptibility will be used. For stainless steels where thecutting debris exhibits little or no magnetic susceptibility aftercutting, an abrasive material having a high magnetic susceptibility,such as a cast steel, stainless steel or shot, will be used. Wherestainless steel cutting debris exhibits high magnetic susceptibility dueto the hardening caused by the cutting process, a garnet, alluvial, orother conventional abrasive material having a low magneticsusceptibility will be used.

The magnetic filter 101 removes the material having a high magneticsusceptibility from the waste stream discharged from the pump module 20.The material having a high magnetic susceptibility may be either theradioactive debris particulate or the abrasive material as describedabove. At periodic intervals, the contents of the magnetic filter 101are purged by de-energizing the magnets 102 of the filter and using airinjection, scrapers, and/or a water flush. The slurry of material havinga high magnetic susceptibility is then discharged from the magneticfilter 101 through a first outlet 103. The remainder of the wastestream, absent the material having a high magnetic susceptibility,passes through and is discharged from the magnetic filter 101 through asecond outlet 104.

A first set of valves 105, 106 are associated with the first outlet 103of the magnetic filter 101, and a second set of valves 107, 108 areassociated with the second outlet 104. The first set of valves 105, 106are used to direct the material having a high magnetic susceptibilitydischarged from the magnetic filter 101 selectively to either a slurrypump 109 for filling a container 110 for storing the abrasive material,or to the concentrate handling module 40. The second set of valves 107,108 are used to direct the material having a low magnetic susceptibilitydischarged from the magnetic filter 101 selectively to either the solidmedia filter module 30, or the slurry pump 109 for filling the container110.

The first and second sets of valves 105-108 are controlled by thecontroller 33 from a remote location based on the type of material beingcut. If the radioactive material being cut has a high magneticsusceptibility, the first set of valves 105, 106 will cause theradioactive debris to pass to the concentrate handling module 40, andthe second set of valves 107, 108 will cause the waterjet abrasivehaving a low magnetic susceptibility to pass to the slurry pump 109 forfilling the container 110. If the radioactive material being cut has alow magnetic susceptibility, the second set of valves 107, 108 willcause the radioactive debris to pass to the solid media filter module30, and the first set of valves 105, 106 will cause the waterjetabrasive to pass to the slurry pump 109 for filling the container 110.

The container 110 for storing the abrasive material is a largepolypropylene high integrity container (HIC) commonly used for bulkdisposal of low level contaminated waste. The container 110 provides arelatively low cost disposal option as compared to the disposal canister51 described above. Other suitable types of containers can also be usedthat meet these criteria.

The magnetic filtration system 100 according to the present inventionprovides several advantages over the existing technology. The magneticfilter 101 separates a substantial portion of the nonradioactiveabrasive particles from the high level radioactive particulate beingcut, thereby significantly reducing the volume and disposal cost of GTCCwaste and allowing disposal of the major portion of the abrasive by lesscostly means (the acronym “GTCC” refers to Greater Than Class “C” wasteas defined by 10 CFR Part 61). The nonradioactive abrasive particles,which represent secondary waste, can be handled and disposed of by moreconventional and less costly methods since the magnetic separationprocess will have removed a majority of the radioactive particulate fromthis waste stream.

The magnetic filter 101 is used in conjunction with a closed-loop, cutzone filtration system to minimize contamination of the general poolarea water, and to minimize the volumes of high and low levelradioactive waste by separating particle sizes.

The abrasive material having a high magnetic susceptibility, when used,can be of a structure and hardness that will minimize the amount offracturing or breakdown (i.e., sufficient hardness to cut yet ductileenough to minimize fracturing) of the abrasive during the cuttingprocess relative to the conventionally used garnet or alluvial material,which experience significant breakdown into small particle sizes. Thelarger particle size of the abrasive material having a high magneticsusceptibility will result in particles that are also more readilycaptured by the solid media filters 31 and/or the cartridge filters 61,62.

The magnetic filter 101 can also be used to capture all of the cuttingwaste (abrasive and radioactive cut material) where differentiationbetween the materials is not possible or practical, thereby minimizingor eliminating the use of consumable cartridge filters that otherwiseresult in increased secondary waste. The use of the magnetic filter 101to capture both the abrasive and radioactive cut material has anadvantage over the use of the solid media filters 31 because themagnetic filter 101 can remove waterjet process fines that are smallerthan the practical size that can be filtered by the solid media filters31 operating as described above.

It will be appreciated that the present invention is not limited to theexact constructions that have been described above and illustrated inthe accompanying drawings, and that various modifications and changescan be made without departing from the scope and spirit of theinvention. It is intended that the scope of the invention only belimited by the appended claims.

What is claimed is:
 1. A method of separating radioactive cut materialfrom nonradioactive abrasive cutting material, comprising the steps of:determining a magnetic susceptibility of a radioactive material to becut; selecting an abrasive cutting material to be used with a waterjetcutting tool such that the abrasive cutting material has a differentmagnetic susceptibility than said radioactive material to be cut;capturing a mixture of radioactive cut material and abrasive cuttingmaterial generated during operation of the watejet cutting tool; andseparating the radioactive cut material from the abrasive cuttingmaterial using a magnetic filter.
 2. The method of claim 1, furthercomprising the steps of directing the abrasive cutting material from themagnetic filter to a storage container for low level contaminated waste,and directing the radioactive cut material from the magnetic filter to astorage canister for high level radioactive waste.
 3. The method ofclaim 1, wherein said step of selecting an abrasive cutting materialcomprises selecting an abrasive cutting material with a high magneticsusceptibility if the radioactive cut material has a low magneticsusceptibility, or selecting an abrasive cutting material with a lowmagnetic susceptibility if the radioactive cut material has a highmagnetic susceptibility.
 4. The method of claim 1, further comprisingthe steps of: placing an enclosure in a fluid volume for isolating aportion of the fluid volume from a remainder of the fluid volume;capturing the mixture of radioactive cut material and abrasive cuttingmaterial within said enclosure; filtering the radioactive cut materialfrom a fluid stream discharged from the magnetic filter; and returningfluid from the fluid stream back into the isolated portion of the fluidvolume.
 5. The method of claim 1, wherein said step of selecting anabrasive cutting material comprises selecting garnet when theradioactive cut material is high alloy carbon steel.
 6. The method ofclaim 1, wherein said step of selecting an abrasive cutting materialcomprises selecting alluvial material when the radioactive cut materialis high alloy carbon steel.
 7. The method of claim 1, wherein said stepof selecting an abrasive cutting material comprises selecting garnetwhen the radioactive cut material is low alloy carbon steel.
 8. Themethod of claim 1, wherein said step of selecting an abrasive cuttingmaterial comprises selecting alluvial material when the radioactive cutmaterial is low alloy carbon steel.
 9. The method of claim 1, whereinsaid step of selecting an abrasive cutting material comprises selectingcast steel having a high magnetic susceptibility if the radioactive cutmaterial is stainless steel exhibiting no magnetic susceptibility. 10.The method of claim 1, wherein said step of selecting an abrasivecutting material comprises selecting stainless steel having a highmagnetic susceptibility if the radioactive cut material is stainlesssteel exhibiting no magnetic susceptibility.
 11. The method of claim 1,wherein said step of selecting an abrasive cutting material comprisesselecting shot having a high magnetic susceptibility if the radioactivecut material is stainless steel exhibiting no magnetic susceptibility.